The present invention relates generally to field-type electron emitters, and, more particularly, to a system for limiting emittance growth in an electron beam. A field emitter unit includes an emittance compensation electrode that functions to minimize degradation of the electron beam and allow for focusing of the electron beam into a desired spot size.
Electron emissions in field-type electron emitter arrays are produced according to the Fowler-Nordheim theory relating the field emission current density of a metal surface to the electric field at the surface. Most field-type electron emitter arrays generally include an array of many field emitter devices. Emitter arrays can be micro- or nano-fabricated to contain tens of thousands of emitter devices on a single chip. Each emitter device, when properly driven, can emit a beam or current of electrons from the tip portion of the emitter device. Field emitter arrays have many applications, one of which is as electron sources in microwave tubes, x-ray tubes, and other microelectronic devices.
The electron-emitting field emitter devices themselves may take a number of forms, such as a “Spindt”-type emitter. In operation, a control voltage is applied across a gating/extraction electrode and substrate to create a strong electric field and extract electrons from an emitter element placed on the substrate. Typically, the gate layer is common to all emitter devices of an emitter array and supplies the same control or emission voltage to the entire array. In some Spindt emitters, the control voltage may be about 100V. Other types of emitters may include refractory metal, carbide, diamond, or silicon tips or cones, silicon/carbon nanotubes, metallic nanowires, carbon fibers, or carbon nanotubes.
When used as an electron source in an x-ray tube application, it is desirable to lower the voltage necessary for the field emitter elements to generate an electron beam, so as to lower the probability of breakdown caused by operational failures and structural wear associated with an overvoltage being applied to the gate layer. Thus, certain mechanisms are employed to lower the voltage needed for extracting an electron beam from the cathode, with one such mechanism being a grid structure. A grid structure functions to enhance the electric field strength at the surface of the emitter element, thus lowering the necessary extraction voltage. However, while the grid mesh significantly improves the extraction efficiency, it also has a negative impact on electron beam quality due to the interaction of the electron beam with the grid. That is, interaction of the electron beam with the grid can increase the degradation of the electron beam quality by increasing beam emittance, which prevents the electron beam from focusing onto a small, useable focal spot on the anode.
Thus, a need exists for a system that minimizes emittance growth in the electron beam due to the extraction grid and is able to achieve continuously controlled beam focusing. It would also be desirable to have a system that allows for modulation of the electron beam current while controlling emittance growth in the electron beam.